If they bought and paid for the item you cannot change it to a postage option now.
Best way would be to cancel the order citing Buyer requests to cancel, relist with only a postage option.
This of course after you message, to explain what you are going to do and that they will need to buy it again from your eBay listing.
You can then after they buy and pay, manually mark the item as dispatched adding that all important tracking number in the same place.
@nicoljoburn-0